International projectionist (Jan-Dec 1935)

Mr. Hoffman Replies: COSTS INITIAL AND OPERATING FAVOR THE A. C. ARC Ernest Hoffman HOFFMAN & SONS ELEC. ENG. CORP. READERS of I. P. might have gathered from the article "A. C. vs. D. C. Suprex Arc Operation," which appeared in these columns last month, that I am opposed to the new D. C. arc. Mr. Finn's reply, directed specifically to me, may have lent color to this belief. Such is not the case. I am not opposed to the new Suprex D. C. arc, as there is no question but that it delivers results comparable with any type of hi-low lamp and, in some instances, approximates the results obtained with the rotating highintensity arc, thus showing considerable savings. Certainly the D. C. arc has a field — and a wide one. I do maintain, however, that an A. C. arc, given a proper controller and suitable transformer, would fill the bill very well. The smaller theatres now using low-intensity lamps (and they certainly constitute a great majority of the projection field) could use the A. C. arc to great advantage, considering the savings made possible by its use. I do not agree with Mr. Finn's analogy of the action of an arc and an incandescent lamp. Flicker is caused by the frequency of the supply circuit, and in support of this reference is made to the action of incandescent lamps on 25-cycle current in which the flicker is visible to the naked eye. This is not the case on 60-cycles, as I pointed out, and certainly this flicker is not visible in the A. C. arc because the gas ball, and not the crater, is the light source — although it is a fact that the crater is necessary to produce this gas ball. Cites D. C. Arc Flicker Shutter flicker we shall have with us until projector manufacturers find some means of eliminating it. This shutter flicker is evident with any light source; and I maintain that the more light thrown on the screen, the more visible becomes the flicker. In proof of this, we found in an actual test that there was more flicker apparent on the screen with the D. C. arc than with the A. C. arc, solely because the former delivers more light to the screen and thus accentuates the flicker effect to the shutter. Relative to conversion jobs, I fail to see where the addition to present equipment of a transformer and a controller makes junk of the installation. Neither do I feel, of course, that the conversion of present equipment which is in bad shape will make possible "Broadway projection." My contention is that the A. C. arc has a definite place in the projection field, particularly where costs are concerned. Relative to the status of rectifiers, let us bear in mind that oscillograph curves used in an article by W. C. Kalb in these columns1 disclose that the A. C. arc curves and the rectified curves for the D. C. arc are practically superimposable upon each other. This bears out my contention that if flicker be visible with the A. C. arc, this condition is aggravated with the D. C. arc which gives more light. Mr. Finn appears to be suffering from a rectifier complex, so vigorously does he insist upon their efficiency. We all know that where rectifier tubes are in use on up to 3,000 hours, these cases are merely the exceptions that prove the rule. Were the efficient operating life of bulbs in excess of 1,000 hours, the manufacturers thereof would certainly raise the guarantee period. As to copper-oxide rectifiers, my experience with the smaller sizes is that trouble did develop in the disks. With the increased current capacity required by the Suprex D. C. arc — 45 amps, or more — this trouble will increase and replacement of units will be necessary, to what extent neither I nor anybody else now knows. Strongly Favors Generators All factors being granted careful consideration, motor generators should be favored for the Suprex D. C. arcs, where '•''Characteristics and Uses of the Carbon Arc," Vol. 7, No. 4, Oct., 1934, pp. 11-15. [18] new installations are contemplated. Costs being a secondary consideration (as they must be in new jobs) I personally would favor generators over any type of rectifier. I am very glad to accept the invita tion to submit tables of costs bearing on (1) new equipment costs, and (2) operating costs. It will be noted that for a majority of theatres during these trying times many factors favor the use of the A. C. arc. A conversion job certainly is in order in all cases where a minimum capital expenditure is essential, provided, of course, that present equipment be in fair condition and an immediate return on the investment is desired. Assuming that present equipment is replaced (as Mr. Finn recommends), the capital investment on the part of the theatre is set forth in Table A. It is apparent therefrom that, based on list prices, from $1100 to $1850 will have to be expended, depending upon the purchaser's choice. This brings us to the really serious blow to the theatre — operating costs. I have prepared a chart, Table B, which sets forth very clearly the various operating costs — for Low-Intensity, for the new A. C. arc, and for the Suprex D. C. arc. It will be noted that, quite apart from the installation expense, the operating costs range from $773.80 per year for Table A NEW INSTALLATION COSTS % 2 Lamps ($300-450) $600 to 900 2 Rheost. (28-37) 56 to 74 1 M. G. Set (aver.) 763 to 876 Total 1108 1568 WITH RECTIFIERS: 2 Rect. ($200-280) * $400 to 560 Bulbs (12) @ $9 ea.* .... 108 to 108 2 Lamps (as above) 600 to 900 Total ....1108 1568 *Mr. Hoffman's original minimum rectifier figure of $160 is too low; while his bulb figure of $15 is too high. — Ed.